November 2008

Rev 2

1/16

AN2678

Application note

Extremely accurate timekeeping over

temperature using adaptive calibration

Introduction

Typical real-time clocks use common 32,768 Hz watch crystals. These are readily available

and relatively inexpensive, but they suffer a loss of accuracy when operated over wide

temperature ranges. However, the ultra-low power characteristics of 32 KHz oscillators

make them ideal, and necessary, for battery backed applications.

Conversely, the higher speed, AT-cut crystals used with microprocessors have low drift over

a wide temperature range, and can thus provide high accuracy, but their oscillators are not

suitable for backup since they will draw much more current at the frequencies common with

AT-cut crystals.

The purpose of this application note is to show how, using a combination of these crystal

characteristics, users can get high accuracy over a wide temperature range using ST's

M41T82-83-93 series of RTCs. This is accomplished by first measuring the frequency and

then using both the analog and digital calibration features of these devices.

Figure 1.

AT versus watch crystal - typical characteristics

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

20

-50 -40 -30 -20 -10 0

10 20 30 40 50 60 70 80 90 100

TEMP (˚C)

Watch crystal

AT cut crystal

ai13944

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